I had the exact same problem as everyone else, but I am using an Asus ASUS RS700-E7/RS8, Here is what solved the problem for me:
See the attached photo, By default EHCI handoff was disabled, so I selected "Enabled" as shown and rebooted.
The keyboard and other USB devices then continued to be responsive. Some research into EHCI handoff is next on my list unless someone can shed some light on this subject.